The present invention relates to manual transmissions for vehicles, and more specifically, to a manual transmission of the type including a countershaft which, under relatively light torques, is able to oscillate due to engine torque pulses transmitted by the input shaft.
Oscillation of the countershaft, under light torque and light acceleration conditions, is considered undesirable because of the possibility of "gear rattle", which can occur between any pair of meshed gears on the countershaft and output shaft through which no substantial amount of torque is being transmitted.
The prior art has attempted to dampen such oscillations of the countershaft, and thereby eliminate gear rattle. U.S. Pat. No. 4,677,868 discloses a manual transmission of the countershaft type in which the head gear and the countershaft are not formed integrally, but instead, the head gear is separate from the countershaft, but includes a set of splines which is in loose splined engagement with a second set of splines, such that the head gear is able to transmit torque to the countershaft through the loose spline connection. In the cited patent, the loose spline connection permits about one degree of relative rotation between the adjacent splines. The spline area is filled with viscous fluid, for the stated purpose of dampening torsional oscillations between the head gear and the countershaft.
In the device of the '868 patent, however, when torque is transmitted to the head gear, under light torque and acceleration conditions, the splines associated with the head gear will engage the adjacent splines associated with the countershaft, with the only dampening being that which occurs as the fluid is pumped from between adjacent splines, as those splines move into engagement. However, with only about one degree of relative rotation possible in the splines, very little dampening can occur.